The development of cigarettes with low smoke yields is an important trend in the cigarette industry in order to reduce the harmful effects of tobacco smoke. This is mainly driven by legal requirements, as in many countries upper limits for the components of the smoke, such as tar, nicotine or carbon monoxide, are in force. For example, in the European Union it is not permissible to produce or sell cigarettes which provide more than 10 mg tar, 1 mg nicotine or 10 mg carbon monoxide in a standardized test. Similar laws exist also in other countries.
A typical cigarette consists of a tobacco rod, which is wrapped by a cigarette paper and is mostly of a cylindrical shape. Additionally, there is a filter at one end, which mostly consists of cellulose acetate and is wrapped with a filter wrapping paper. Additionally to pure cellulose acetate filters, there is the possibility to employ segmented filters. These comprise one or more segments, in which different substances, such as activated carbon or paper filters, are located. The filter and the tobacco rod, wrapped with the cigarette paper are connected to each other by a tipping paper. Furthermore, it is known to perforate the tipping paper in order to dilute the smoke, which flows through the filter.
The smoke yields of a cigarette can be determined by means of a procedure according to ISO 4387. Here, a cigarette is lit at the first puff and after that a puff is taken each minute with a puff duration of 2 seconds and a volume of 35 cm3 with a sinus-shaped puff profile. This is repeated until the cigarette falls below a length defined in the standard. The smoke exiting from the mouth end of the cigarette is collected in a Cambridge Filter Pad, which is analyzed afterwards to determine the amount of tar and nicotine and, if needed, the content of various other substances. The gaseous components which are not absorbed in the filter are passed on and are also analyzed, for example to determine the content of CO.
The tobacco rod of a cigarette is wrapped with a cigarette paper, which at least partially consists of cellulose fibers, for example wood pulp fibers or fibers from flax, hemp or sisal.
The wood pulp fibers used for paper production are usually differentiated into long and short fibers, wherein the long fibers are typically cellulose fibers from coniferous wood, such as spruce or pine, with a length of more than 2 mm, while the short fibers originate generally from deciduous trees, such as birch, beech or eucalyptus, and typically have a length of less than 2 mm, often of about 1 mm.
The cellulose fibers typically account for about 60 to 100% by weight of the finished paper. The cigarette paper can also contain filler materials, wherein mainly chalk is used, but also other inorganic fillers are possible filler materials, such as titanium dioxide, calcium sulfate, magnesium carbonate, magnesium oxide, magnesium hydroxide, aluminum hydroxide and talc. The mass share of the inorganic fillers amounts typically to up to 40% by weight of the finished paper.
In addition the cigarette paper can contain substances which control the smoldering behavior of a cigarette. Examples are sodium and potassium citrates, sodium and potassium hydrogen carbonates, ammonium, sodium, and potassium acetates, and sodium and potassium salts of formic acid, malic acid, lactic acid and ammonium, sodium and potassium phosphates, which are added at a mass share of up to 5% by weight. Furthermore, the addition of aromatic substances is possible in order to adjust the taste of a cigarette manufactured from this cigarette paper or to aromatize the side-stream smoke.
An important parameter of a cigarette paper is its diffusivity. The diffusivity is a measure for the gas flow through the cigarette paper caused by a concentration difference. It therefore indicates the gas volume passing through the paper per unit of time, per unit of area and per concentration difference and thus has the unit cm3/(cm2 s)=cm/s. A measurement of the CO2 diffusivity can be carried out, for example, with a diffusion measurement instrument of the companies Borgwaldt KC (diffusivity tester) or Sodim (CO2 diffusivity meter).
The measurement of diffusivity can take place under standard conditions according to ISO 187. Additionally, the cigarette paper can also be exposed to an elevated temperature to simulate a thermal stress. A possible approach is to expose the cigarette paper for 30 minutes to a temperature of 230° C. in the presence of air. This can be done in a common drying oven. Since the change in diffusivity of the cigarette paper due to the heating process is irreversible, the cigarette paper can be conditioned to the standard conditions according to ISO 187 after heating, before the measurement of diffusivity is performed. In the present disclosure all data on diffusivity are values which were obtained in accordance with this procedure, that is to say, after heating to 230° C. for 30 minutes and subsequent conditioning in accordance with ISO 187.
To reduce the amount of substances harmful to health in the smoke of a cigarette, several approaches are known from the prior art. One possibility is to dilute the smoke flowing through the cigarette by allowing an inflow of air. This is called ventilation. Increased ventilation leads to a stronger dilution of the main-stream smoke and consequently to lower smoke yields. The ventilation of a cigarette can be adjusted, for example, by a perforation on the tipping paper or by the air permeability of the cigarette paper.
A further option to adjust the smoke yields is the filtration of the main-stream smoke. This can be implemented, for example, by a filter made from cellulose acetate or by segmented filters. The latter, besides one or more segments made of cellulose acetate, also have chambers, which are filled with certain substances, for example activated carbon or paper filters. These cause additional filtration of the smoke and thus lead to a reduction of smoke yields, and sometimes also to a selective reduction of certain smoke components.
A further option to reduce the smoke yields consists in replacing some of the tobacco by a different material, which can be non-combustible in part, and therefore fewer harmful substances are generated on the whole. Such a material is described in K G. McAdam et al, The use of a novel tobacco substitute-sheet and smoke dilution to reduce toxicant yields in cigarette smoke, Food and Chemical Toxicology, Volume 49, Issue 8, 1684-1696 (2011). This is a material which consists largely of glycerin and sodium alginate.
On the whole, it is sought to achieve a reduction of smoke yields, preferably even a selective reduction of certain smoke components, but without causing a substantial change in the overall characteristic of the cigarette from the smoker's viewpoint. In particular, measures that negatively influence the customer acceptance or the taste of the cigarette, as is the case for example with high ventilation or strong filtration, are undesirable. Also, the replacement of tobacco by alternative materials has disadvantages. On the one hand such substitute materials can often be processed only poorly on conventional cigarette machines. On the other hand substances which are typically not present or are not present in the same ratio in tobacco or the cigarette paper are often included in the tobacco rod as a result of these substitute materials, which negatively influences the taste of such cigarettes.